As Chris Hemsworth reprises his role as the God of Thunder in “Thor: The Dark World,” we ask his trainer, Michael Knight, about how he stays fit and strong.

"Once you get an actor to their desired shape, the tricky part is keeping them there — a lot of people tend to slack off,” says Michael Knight, who’s been training Chris Hemsworth on and off for years. Yup, even the star of the new “Thor: The Dark World” has a hard time maintaining that god bod.

“Chris is in and out of movies, so the hardest part for me is making him lose weight quickly, and then getting him to put it back on without his body looking like hell,” says Knight. Hemsworth lost 30 pounds to play a Formula 1 driver in “Rush,” and then had to put it right back on in time for “Thor.” "So I always try and simulate the same workout in order to maintain his body shape in the long term.”

The maintenance workout — designed by Knight just for Hemsworth — comprises three 20-minute sessions, which the star does up to five times a week.

“My system is effective, and means you don’t have to go out and do things that are going to tear up your body — like too much running or heavy weights,” Knight says of his regime, which he adds “starts off slow and finishes with a fury.”

“On some days, you’re going into the time-under-tension method, which is where you’re using really heavy weights to tire your core, and kettlebells. On other days, you might be doing double kettlebells or working with a heavy tire, and finishing with a cardio blast. Whatever you do, each 20-minute session will leave you pretty shot.”

The Chris Hemsworth workout

If, like Hemsworth, you’re already in shape, then this workout will keep you strong. And it’s all about big lifts. “The idea is that you continually rotate body parts: One day you work your chest, the next day your back, then your shoulders, and then you go back to your chest and so on.

"But you’ll always be working the hips and the legs — they’re the largest muscles in the body, and unless they get stronger, nothing else can,” says Knight. You’re sometimes going to hit the same part twice in that week, sometimes once, but you’re always going to start fresh again on Monday.

Unless you’re auditioning for “Thor 3,” you don’t need to train exactly like Hemsworth. But this abbreviated version should still do the trick. Here’s an overview of what your week will entail:

When it comes to food, you need structure. Knight suggests keeping your diet clean by sticking to complex carbs, lean protein and healthy fats. “Eat for fuel, not for enjoyment. But enjoy what you eat,” he says. Knight recommends: